Fabrication method of nitride semiconductors and nitride semiconductor structure fabricated thereby

ABSTRACT

Disclosed is a method of fabricating nitride semiconductors in a MOCVD reactor. GaN is first deposited on an inner wall of the MOCVD reactor, and a sapphire substrate is loaded into the MOCVD reactor. The sapphire substrate is heated and etching gas is injected into the MOCVD reactor. NH 3  gas is injected into the MOCVD reactor to nitrify the surface of the sapphire substrate. A nitride semiconductor layer is grown on the nitrified sapphire substrate. By surface-reforming the sapphire substrate and then growing the nitride semiconductor layer on the surface-reformed sapphire substrate via MOCVD without formation of a low temperature buffer layer, an excellent nitride semiconductor structure can be realized. In this circumstance, the nitride semiconductor layer for example of GaN can be grown effectively on the surface-treated sapphire substrate because GaN deposition occurs on the sapphire substrate while it is etched.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride semiconductor. Moreparticularly, the present invention relates to a fabrication method ofnitride semiconductors, by which a sapphire substrate issurface-reformed to grow a nitride semiconductor layer on the sapphiresubstrate via Metal-Organic Chemical Vapor Deposition (MOCVD) withoutformation of a low temperature buffer layer, and a nitride semiconductorfabricated thereby.

2. Description of the Related Art

A Light Emitting Diode (LED) as a kind of optoelectric device basicallyconsists of a junction of p- and a n-doped semiconductor layers. Whenapplied with electric current, the LED converts energy corresponding toits band gap into light through electron-hole combination. Recently,full color LEDs are produced based upon GaN compounds to emit blue,green and ultraviolet lights.

A GaN LED has a drawback that a GaN layer grown directly on a sapphire(Al₂O₃) substrate forms an opaque layer therebetween because GaN poorlywets with sapphire owing to the lattice constant mismatch.

A solution to this problem is disclosed in the U.S. Pat. No. 5,290,393,entitled “Crystal Growth Method for Gallium Nitride-Based CompoundSemiconductor.” This solution proposes to fabricate a GaN-basedsemiconductor by growing a buffer layer of nitride compound such as GaN,AlN and InN on a sapphire substrate at a low temperature of about 500°C. and then a GaN layer on the buffer layer at a high temperature.

A low temperature GaN buffer layer grown to a thickness of 20 to 30 nmis polycrystalline or has a partial metastable cubic structure. Aftergrowth of the low temperature buffer layer, temperature elevation causesGaN compound to crystallize transforming its phase partially into astable hexagonal structure and the surface morphology of GaN compoundalso changes coarse forming islands. As a hot temperature GaN layer isformed on the buffer layer, the islands of the low temperature bufferlayer begin to coalesce together thereby forming a fine GaN layer.

However, this solution has a drawback that surface defects such asstacking faults and threading dislocations may occur during coalescenceof the islands.

In the meantime, Korean Patent Application Publication No.2000-0055374and the U.S. Pat. No. 6,528,394 claiming its priority, entitled “GrowthMethod of Gallium Nitride Film”, propose a GaN layer growth method viaHydride Vapor Phase Epitaxy (HVPE).

According to this document, a fine GaN layer is formed on a sapphiresubstrate through first nitrification, pre-treatment with ammonia (NH₃)and HCl gas, second nitrification and hot GaN film formation. In thiscircumstance, the term “nitrification” refers to unintentional formationof a thin film for example of AlN on the sapphire by flowing NH₃ gasonto the sapphire substrate but not to intentional formation of an AlNbuffer layer.

This method is advantageous for growing a relatively thin GaN layer ofabout 100 to 200 μm thanks to high GaN growth rate of HVPE. However, inLED fabrication requiring a relatively thin GaN layer of about 3 to 5μm, there is a drawback that it is difficult to control film thicknessor ensure film quality.

Furthermore, although HVPE can advantageously form undoped and n-dopedGaN layers on a sapphire substrate, it is necessary to load the sapphiresubstrate again into a Metal-Organic Chemical Vapor Deposition (MOCVD)reactor after the growth of the GaN layer to form an active region andp-doped GaN layers on the sapphire substrate to fabricate an LED.

SUMMARY OF THE INVENTION

The present invention has been made to solve the foregoing problems ofthe prior art.

It is therefore an object of the present invention to provide afabrication method of nitride semiconductors by which a sapphiresubstrate is surface-reformed to grow a nitride semiconductor layer onthe sapphire substrate via MOCVD without formation of a low temperaturebuffer layer.

It is another object of the present invention to provide a nitridesemiconductor structure having a nitride semiconductor layer that isgrown on a sapphire substrate via MOCVD without formation of a lowtemperature buffer layer.

According to an aspect of the invention for realizing the objects of theinvention, there is provided a method of fabricating nitridesemiconductors in a Metal-Organic Chemical Vapor Deposition (MOCVD)reactor, the method comprising the following steps of:

-   -   (a) depositing GaN on an inner wall of the MOCVD reactor;    -   (b) loading a sapphire substrate into the MOCVD reactor;    -   (c) heating the sapphire substrate and injecting etching gas        into the MOCVD reactor; and    -   (d) injecting NH₃ gas into the MOCVD reactor to nitrify the        surface of the sapphire substrate.

In the method of fabricating nitride semiconductors, the step (c)irregularly etches the surface of the sapphire and re-deposits the GaNonto the surface of the sapphire from the inner wall of the MOCVDreactor, wherein the GaN re-deposited onto the sapphire is preferablyamorphous or polycrystalline.

In addition, the method of fabricating nitride semiconductors mayfurther comprise the step of growing a nitride semiconductor layer onthe nitrified surface of the sapphire substrate after the step (d).

According to another aspect of the invention for realizing the objectsof the invention, there is provided a method of fabricating nitridesemiconductors in a Metal-Organic Chemical Vapor Deposition (MOCVD)reactor, the method comprising the following steps of:

-   -   (a) loading a sapphire substrate into the MOCVD reactor;    -   (b) heating the sapphire substrate and injecting Tri-Methyl        Gallium (TMG) or Tri-Ethyl Gallium (TEG) on a mixed gas        containing NH₃ gas and etching gas into the MOCVD reactor; and    -   (c) injecting the NH₃ gas into the MOCVD reactor to nitrify the        surface of the sapphire substrate.

In the method of fabricating nitride semiconductors, the step (b)irregularly etches the surface of the sapphire substrate and depositsGaN on the surface of the sapphire substrate.

In addition, the method of fabricating nitride semiconductors mayfurther comprise the step of growing a nitride semiconductor layer onthe surface of the nitrified surface of the sapphire substrate after thestep (c).

In the afore-described methods of fabricating nitride semiconductors,the NH₃ gas may be replaced by one selected from a group consisting oftertiary-butylamine (N(C₄H₉)H₂), phenylhydrazine (C₆H₈N₂) anddimethylhydrazine (C₂H₈N₂).

In the afore-described methods of fabricating nitride semiconductors,the sapphire substrate may be replaced by one selected from a groupconsisting of a silicon carbide (SiC) substrate, an oxide substrate andcarbide substrate.

According to yet another aspect of the invention for realizing theobjects of the invention, there is provided a nitride semiconductorstructure produced via MOCVD, comprising: a sapphire substrate with anupper surface etched by etching gas, deposited with GaN and nitrified;and a GaN semiconductor substrate formed on the semiconductor substrate.

In the nitride semiconductor structure, the sapphire substrate may bereplaced by one selected from a group consisting of a silicon carbide(SiC) substrate, an oxide substrate and carbide substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a flowchart illustrating a fabrication method of nitridesemiconductors according to a first embodiment of the invention;

FIG. 2 is a flowchart illustrating a fabrication method of nitridesemiconductors according to a second embodiment of the invention;

FIG. 3 is a sectional view illustrating a sapphire substrate which issurface-treated according to the invention;

FIG. 4 is an AFM photograph of FIG. 3;

FIG. 5 is a sectional view illustrating a nitride semiconductorstructure having a nitride semiconductor layer grown on the sapphiresubstrate shown in FIG. 3;

FIG. 6 is a sectional view illustrating a sapphire substrate which issurface-treated according to the invention;

FIG. 7 is a photograph illustrating a semiconductor structure accordingto the invention;

FIG. 8 is a photograph illustrating a semiconductor structure having asapphire substrate only nitrified according to the prior art; and

FIG. 9 is a photograph illustrating a semiconductor structure having asapphire substrate etched with HCl according to the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is aimed to fabricate nitride semiconductorswithin a Metal-Organic Chemical Vapor Deposition (MOCVD) reactor.

Hereinafter a fabrication method of nitride semiconductors according toa first embodiment of the invention will be first described withreference to a flowchart shown in FIG. 1.

According to the first embodiment of the invention, GaN is deposited onthe inner wall of a MOCVD reactor in S101, and a sapphire (Al₂O₃) isloaded into the MOCVD reactor in S102 unlike the prior art.

In S101, Ga is injected into the MOCVD in a gas containing NH₃ in theform of Tri-Methyl Gallium (TMG) or Tri-Ethyl Gallium (TEG) to depositGaN on the inner wall of the MOCVD reactor.

In S103, the sapphire substrate is heated and etching gas such as HCl isinjected into the MOCVD reactor. Then, HCl etches irregularly thesurface of the sapphire substrate while etching GaN deposited on theinner wall of the MOCVD so that etched GaN is peeled off from the innerwall of the reactor to re-deposit on the etched surface of the sapphiresubstrate. That is, the sapphire substrate is surface-treated orsurface-reformed through above etching and GaN re-deposition. In thiscase, because the re-deposited GaN is amorphous or polycrystalline andhas a high wettablity with a GaN semiconductor layer which will be grownthereon in S105 below, it can function as an excellent buffer betweenthe GaN semiconductor layer and the underlying sapphire substrate.

In S104, NH₃ gas is injected into the MOCVD reactor to nitrify thesurface of the sapphire substrate which was surface-reformed in S103above. In this step, NH₃ can be replaced by at least one selected from agroup including tertiary-butylamine (N(C₄H₉) H₂), phenylhydrazine(C₆H₈N₂) and dimethylhydrazine (C₂H₈N₂).

Then, in S105, a nitride semiconductor layer is grown on the nitrifiedsapphire substrate to realize a desired nitride semiconductor structureof the invention.

Alternatively, the sapphire substrate used in the first embodiment ofthe invention can be replaced by one selected from a group including asilicon carbide (SiC) substrate, an oxide substrate and a carbidesubstrate.

Application of the first embodiment of the invention to actual MOCVDoperation has the following advantages: GaN is generally deposited onthe inner wall of the MOCVD reactor in a fabrication process of nitridesemiconductors. Therefore, without cleaning the MOCVD reactor after thefabrication process of nitride semiconductors, the GaN deposited on theinner wall of the MOCVD reactor can be used as a Ga source tosurface-reform the sapphire substrate. That is, the GaN deposition stepcan be omitted by using the residual GaN formed in the conventionalnitride semiconductor fabrication process.

Hereinafter a fabrication method of nitride semiconductors according toa second embodiment of the invention will be described with reference toa flowchart shown in FIG. 2.

According to the second embodiment of the invention, a sapphiresubstrate is loaded into a MOCVD reactor in S201.

In S202, the sapphire substrate loaded into the MOCVD reactor is heatedand Tri-Methyl Gallium (TMG) or Tri-Ethyl Gallium (TEG) is injected intothe MOCVD reactor on a mixed gas containing NH₃ and etching gas unlikethe prior art.

Then, HCl irregularly etches the surface of the sapphire substrate whileGaN is deposited on the etched surface of the sapphire substrate so thatthe sapphire substrate is surface-treated or reformed through etchingand GaN deposition. In this case, GaN deposit or a GaN deposit layer onthe sapphire substrate has an excellent wettability with a GaNsemiconductor layer which will be grown thereon in S204 below, it caneffectively function as a buffer between the GaN semiconductor layer andthe underlying sapphire substrate.

In S203, NH₃ gas is injected into the MOCVD reactor to nitrify thesurface of the sapphire substrate which was surface-reformed in S202above. In this step, NH₃ can be replaced by at least one selected from agroup including tertiary-butylamine (N(C₄H₉) H₂), phenylhydrazine(C₆H₈N₂) and dimethylhydrazine (C₂H₈N₂).

Then, in S204, a nitride semiconductor layer is grown on the nitrifiedsapphire substrate to realize a desired nitride semiconductor structureof the invention.

Alternatively, the sapphire substrate used in the second embodiment ofthe invention can be replaced by one selected from a group including aSiC substrate, an oxide substrate and a carbide substrate.

Hereinafter a nitride semiconductor substrate obtained according to theafore-described fabrication method of nitride semiconductors of theinvention will be described with reference to FIGS. 3 to 6.

First, FIG. 3 illustrates a surface-reformed sapphire substrate 12resulting from the afore-described fabrication method, in which itssurface is etched with HCl and a buffer layer 14 is deposited with GaNand nitrified.

An AFM photograph in FIG. 4 more specifically illustrates the surfacestatus of the sapphire substrate 12 in FIG. 3. The surface of thesubstrate 12 is etched to form a number of pillar-shaped projections asshown in FIG. 4, which facilitate deposition of GaN readily on thesubstrate surface.

In a following process, a GaN semiconductor layer 16 is grown on thesapphire substrate 14 to obtain a nitride semiconductor structure 10 ofthe invention as shown in FIG. 5.

FIG. 6 is a sectional view illustrating the surface structure of asapphire substrate which is obtained according to the prior artdisclosed in Korean Patent Application Publication No.2000-0055374 andthe U.S. Pat. No. 6,528,394.

In the prior art, a buffer layer 104 for example of AlN is formed on thesapphire substrate 102 through first nitrification, surface-treatmentwith a mixed gas of NH₃ and HCl and then second nitrification. In moredetail, the buffer layer 104 is formed by first nitrifying the substrate102 to form a thin AlN layer on the surface of the substrate 102,etching the nitrified substrate 102, and second nitrifying the etchedsubstrate 102.

Unlike the conventional process, the invention performs no pre-treatmentto the surface of the substrate 12 but injects HCl and NH₃ into theMOCVD reactor while operating the MOCVD reactor so that GaN peels offfrom the inner wall of the MOCVD reactor and re-deposits on the uppersurface of the substrate 12 to form the buffer layer 14 of the invention(First Embodiment). Alternatively, the interference layer 14 of theinvention can be formed by injecting Ga in the form of TMG or TEG on amixed gas of NH₃ and HCl into the MOCVD reactor while operating theMOCVD reactor so that GaN is deposited on the upper surface of thesubstrate 12 (Second Embodiment).

According to the invention, unlike the prior art, the buffer layer 14 onthe substrate is formed of GaN compound through deposition, which ismore advantageous for growing a GaN semiconductor layer thereon in thefollowing step.

Therefore, it can be understood that the buffer layer 14 of theinvention is advantageous for growth of a GaN semiconductor layerthereon because it is formed through deposition of GaN unlike the priorart.

Further, the invention can carry out the entire semiconductorfabrication process in a single MOCVD reactor without drawing thesapphire substrate out of the MOCVD reactor after GaN deposition becauseGaN is deposited at a relatively high temperature via MOCVD.

EXAMPLE

Sapphire substrates were surface-treated according to the firstembodiment of the invention and the prior art, and their X-raydiffraction data are reported in Table 1 below. TABLE 1 X-RayDiffraction Data of Substrate Surface (unit:arcsec) Classification (002)XRD FWHM (102) XRD FWHM Inventive Example  432  540 Comparative Example1 1266 1857 2 1112 1850 3  966 1217

Above Diffraction Data report values of Full Width at Half Maximum(FWHM) in (002) and (102) orientations of sapphire substrates ofinventive and comparative examples to compare properties thereof.

Inventive Example formed an interference layer through NH₃ nitrificationand GaN deposition simultaneous with HCl etching according to the firstembodiment of the invention. On the other hand, nitrification isaccompanied with etching in Comparative Example 1, only nitrification iscarried out in Comparative Example 2, and etching and followingnitrification were carried out in Comparative Example 3.

As can be seen from Table 1 above, it was observed that InventiveExample has the smallest XRD FWHM and thus the best crystallinity.

Hereinafter such crystallity will be discussed with reference to FIGS. 7to 9: FIG. 7 is a photograph of a nitride semiconductor structure whichwas surface-reformed according to the invention, in which a backgroundblack portion is seen through a central circle of a transparent mirrorsurface formed on the nitride semiconductor structure revealing. FIG. 8is a photograph of a nitride semiconductor structure of the prior art,in which a GaN semiconductor layer was grown on a sapphire substrateonly after nitrification of the sapphire substrate. This nitridesemiconductor structure is opaque and thus shows poor transparency. FIG.9 is a photograph of another semiconductor structure of the prior art,in which a GaN semiconductor layer was grown on an HCl-etched sapphiresubstrate. This nitride semiconductor structure is also opaque and thusshows poor transparency.

As set forth above, the invention can surface-reform a sapphiresubstrate and then grow a nitride semiconductor layer on thesurface-reformed sapphire substrate via MOCVD without formation of a lowtemperature buffer layer to realize an excellent nitride semiconductorstructure. In this circumstance, the nitride semiconductor layer forexample of GaN can be grown effectively on the surface-treated sapphiresubstrate because GaN deposition occurs on the sapphire substrate whileit is etched.

Further, because GaN which is deposited on the inner wall of the MOCVDreactor in the prior step can be utilized as a nitride source, theinvention can have an effect of reducing a process step thereby to saveraw material and cost.

Further, the invention has an advantage that the entire process can becarried out in a single MOCVD reactor.

While the present invention has been shown and described in connectionwith the preferred embodiments, it will be understood by those skilledin the art that various modifications and variations can be made withoutdeparting from the spirit and scope of the invention as defined by theappended claims.

1. A method of fabricating nitride semiconductors in a Metal-OrganicChemical Vapor Deposition (MOCVD) reactor, the method comprising thefollowing steps of: (a) depositing GaN on an inner wall of the MOCVDreactor; (b) loading a sapphire substrate into the MOCVD reactor; (c)heating the sapphire substrate and injecting etching gas into the MOCVDreactor; and (d) injecting NH₃ gas into the MOCVD reactor to nitrify thesurface of the sapphire substrate.
 2. The method of fabricating nitridesemiconductors according to claim 1, wherein the step (c) irregularlyetches the surface of the sapphire and re-deposits the GaN onto thesurface of the sapphire from the inner wall of the MOCVD reactor.
 3. Themethod of fabricating nitride semiconductors according to claim 2,wherein the GaN re-deposited onto the sapphire is amorphous orpolycrystalline.
 4. The method of fabricating nitride semiconductorsaccording to claim 1, further comprising the step of growing a nitridesemiconductor layer on the nitrified surface of the sapphire substrateafter the step (d).
 5. The method of fabricating nitride semiconductorsaccording to claim 1, wherein the sapphire substrate is replaced by oneselected from a group including a silicon carbide (SiC) substrate, anoxide substrate and carbide substrate.
 6. The method of fabricatingnitride semiconductors according to claim 1, wherein the NH₃ gas isreplaced by one selected from a group consisting of tertiary-butylamine(N(C₄H₉)H₂), phenylhydrazine (C₆H₈N₂) and dimethylhydrazine (C₂H₈N₂). 7.A method of fabricating nitride semiconductors in a Metal-OrganicChemical Vapor Deposition (MOCVD) reactor, the method comprising thefollowing steps of: (a) loading a sapphire substrate into the MOCVDreactor; (b) heating the sapphire substrate and injecting Tri-MethylGallium (TMG) or Tri-Ethyl Gallium (TEG) on a mixed gas containing NH₃gas and etching gas into the MOCVD reactor; and (c) injecting the NH₃gas into the MOCVD reactor to nitrify the surface of the sapphiresubstrate.
 8. The method of fabricating nitride semiconductors accordingto claim 7, wherein the step (b) irregularly etches the surface of thesapphire substrate and deposits GaN on the surface of the sapphiresubstrate.
 9. The method of fabricating nitride semiconductors accordingto claim 7, further comprising the step of growing a nitridesemiconductor layer on the surface of the nitrified surface of thesapphire substrate after the step (c).
 10. The method of fabricatingnitride semiconductors according to claim 7, wherein the NH₃ gas isreplaced by one selected from a group consisting of tertiary-butylamine(N(C₄H₉)H₂) phenylhydrazine (C₆H₈N₂) and dimethylhydrazine (C₂H₈N₂). 11.The method of fabricating nitride semiconductors according to claim 7,wherein the sapphire substrate is replaced by one selected from a groupconsisting of a silicon carbide (SiC) substrate, an oxide substrate andcarbide substrate.
 12. A nitride semiconductor structure produced viaMetal-Organic Chemical Vapor Deposition (MOCVD), comprising: a sapphiresubstrate with an upper surface etched by etching gas, deposited withGaN and nitrified; and a GaN semiconductor structure formed on thesemiconductor substrate.
 13. The nitride semiconductor structureaccording to claim 12, wherein the sapphire substrate is replaced by oneselected from a group consisting of a silicon carbide (SiC) substrate,an oxide substrate and carbide substrate.